Modern communication is increasingly based on optical networks. The complexity of emerging networks required in internet applications, for example, with tens, if not hundreds, of channels, demands integrated optical devices produced on materials such as silicon using tools and techniques from the semiconductor industry, as opposed to devices fabricated from discrete optical components. This is demonstrated by Meint K. Smit in his article, “Advanced components for WDM,” ECOC'99, 26–30 September, Nice, France, v. I, pp. 98–99, included herein by reference. In this article, Smit presents Arrayed Waveguide Gratings (AWGs), also known as Phase Arrays (PHASARs) or Waveguide Grating Routers (WGRs), whose purpose is to perform filtering of a selected wavelength of an optical spectrum. In these devices, light is directed via an input strip waveguide to a lens region which distributes it among a large number of individual curved strip optical waveguides that make up the grating array. These optical waveguides are fabricated so that each has a precise difference of length relative to neighboring waveguides, thereby producing a phase delay in the light emerging therefrom. When all the emerging light is recombined by a second lens region which focuses it onto a second, output, waveguide array, the superposition of the phase-shifted light beams for different wavelengths results in light being sorted by wavelength among the waveguides of the output array. Currently produced devices filter optical radiation into 64 channels spaced apart by 0.4 nm or 50 GHz, with 128 or even 256 channel devices under development.
It is worth noting, however, that the manufacturing tolerances for a multiplicity of curved strip optical waveguides are highly demanding even by the standards of the semiconductor industry and put severe constraints on the choice of materials for such devices and on their physical dimensions. For example, the index of refraction must be uniform to within 100 ppm. Devices produced from silica on silicon have small losses for optical coupling with optical fibers, but the index of refraction for the waveguides is in a range that requires large size devices, on the order of 10 cm, in order to produce the required phase differences. Semiconductors, such as InP, allow devices on the order of 1 cm, but are difficult to couple with optical fibers. Thus, these devices are large or complex, and are costly. Further limitations are that they are not tunable with respect to the selected optical wavelengths, and that they require two AWG stages to produce full add/drop optical filter functionality.
U.S. Pat. No. 6,141,467 to Doerr discloses a “Wavelength-Division-Multiplexing Programmable Add/Drop Using Interleave-Chirped Waveguide Grating Router,” included herein by reference, that has many of the features described by Smit, but is also tunable by means of phase shifters which selectably change the index of refraction in a portion of the strip waveguides. However, except for the addition of tunability and other additional features, the disclosed device still has the other disadvantages of the devices described by Smit.
U.S. Patent to Herrmann discloses a “Polarization-Independent, Tunable, Integrated Acousto-Optical Waveguide Device for the Wavelength Selection of an Optical Signal” in which wavelength filtering of the optical input to the device is accomplished through acousto-optic conversion, by a surface acoustic wave (SAW), of the different polarization modes of the light propagating collinearly with the SAW in the strip optical waveguide. The add/drop optical filter functionality is provided by polarizing beam splitters which are based on the coupled strip optical waveguides. However, the device disclosed cannot provide narrow enough linewidth due to the value of optical anisotropy and length of interaction area, without significantly increasing the device size. For example, an acousto-optic tunable filter produced on a lithium niobate substrate that is 6.8 cm long has a linewidth of about 2 nm (on a level 0.5) for a wavelength of about 1.5 microns. The device is further limited in that it cannot reassign different optical wavelengths to different optical channels.
A more advanced “Acoustooptic Tunable Filter” has been proposed by the present inventor in PCT application PCT/RU01/00160, included herein by reference, which includes a straight input strip optical waveguide with a multiplicity of tilted reflectors to expand the input beam into a number of reflected beams in a planar or film waveguide, which are then recombined by means of a second set of tilted reflectors along a straight output strip optical waveguide. Surface acoustic waves (SAW) of varying frequency are excited in the planar waveguide which are non-collinear with the beams therein and which periodically vary the index of refraction of the planar waveguide, thereby selecting which specific wavelengths of the optical beams are recombined along the output strip optical waveguide to be passed by the filter. However, the planar waveguide configuration reduces the potentially high efficiency of the filtered optical beam due to the inherent inefficiency of the acousto-optic interaction, requiring high power consumption, and due to the loss of optical energy in the spacing between channels and reflectors that is inherently part of a planar waveguide, so that it never reaches the output strip optical waveguide.
U.S. Pat. No. 5,559,906 to Maerz discloses an “Optical Arrangement of a Strip-shaped Optical Waveguide,” included herein by reference, which includes a number of curved strip waveguides of different lengths of which one or more have a controllable phase shifting arrangement. Phase shifting is accomplished by changing the index of refraction of portions of the curved strip waveguides by one of the following: the electro-optical effect, charge carrier injection, or the thermo-optical effect. In order to vary the phase shift from channel to channel, the electrodes used to induce the change in the index of refraction of the strip waveguides that make up the channels differ in optical length from channel to channel. As with Smit above, the manufacturing tolerances for a multiplicity of curved strip optical waveguides are highly demanding even by the standards of the semiconductor industry, and the addition of individual electrodes of precise, different lengths for each strip waveguide makes this requirement even more demanding. Further, in the disclosed arrangement, beam expansion to distribute beams among the channels and beam recombination from the channels is accomplished by means of a film or planar waveguide (See FIGS. 2 and 3 and claims 8 and 9, therein) which requires a large device size to eliminate the problem of cross talk between channels.